The currently available zcatalog text indexes are temperamental about the search expressions they accept,
and blow up with an error if you enter quotes, too many wildcards, non-ascii characters, the word not, etc. etc.

Thanks Stephan. 1036 is an overview issue, so could you open a new issue specifically for the umlaut searching problem, and provide full details of your setup. Here are some things that will be useful: your zope version, your platform, the kind of catalog index you are using, how you have configured it, whether you are using plone or not, whether you can reproduce the problem on zwiki.org or plone.demo.zwiki.org. Thanks!